linux-kernel - Re: cfq-iosched: tiobench regression

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Date:	Mon, 28 Dec 2009 10:02:03 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>
Subject: Re: cfq-iosched: tiobench regression

On Fri, Dec 25, 2009 at 06:16:27PM +0800, Corrado Zoccolo wrote:
> Hi Shaohua,
> On Thu, Dec 24, 2009 at 1:55 AM, Shaohua Li <shaohua.li@...el.com> wrote:
> > df5fe3e8e13883f58dc97489076bbcc150789a21
> > b3b6d0408c953524f979468562e7e210d8634150
> > The coop merge is too aggressive. For example, if two tasks are reading two
> > files where the two files have some adjecent blocks, cfq will immediately
> > merge them. cfq_rq_close() also has trouble, sometimes the seek_mean is very
> > big. I did a test to make cfq_rq_close() always checks the distence according
> > to CIC_SEEK_THR, but still saw a lot of wrong merge. (BTW, why we take a long
> > distence far away request as close. Taking them close doesn't improve any thoughtput
> > to me. Maybe we should always use CIC_SEEK_THR as close criteria).
> Yes, when deciding if two queues are going to be merged, we should use
> the constant CIC_SEEK_THR.
Ok, will prepare a seperate patch for this.

> > So sounds we need make split more aggressive. But the split is too lazay,
> > which requires to wait 1s. Time based check isn't reliable as queue might not
> > run at given time, so uses a small time isn't ok.
> 1s is too much, but I wouldn't abandon a time based approach. To fix
> the problem of queue not being run, you can consider a slice. If at
> the end of the slice, the queue is seeky, you split it.
Sounds good, will take this way.

Thanks,
Shaohua
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